What is the California Bearing Ratio (CBR)?

The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%).

It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm (0.75 in.) (AASHTO, 2000[1]).

The CBR was developed by the California Division of Highways circa 1930 and was subsequently adopted by numerous states, U.S. federal agencies and internationally. As a result, most agency and commercial geotechnical laboratories in the U.S. and Canada are equipped to perform CBR tests.

The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05″) per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.).

Figure 1.0 is a sketch of a typical CBR sample.

FIGURE 1.0

 

Equation

Values obtained are inserted into the following equation to obtain a CBR value:

Typical Values

Table 1.0 summarizes typical CBR values by soil type.

TABLE 1.0

Related Reading: How does ionic soil stabilization work?

RoadPacker soil stabilization solutions significantly increase CBR values for all soil types, along with drastically decreasing the ability for water to re-enter the soil.Learn more.